Electrolytic treatment of tin plate for preventing sulphur staining



Patented July 29, 1947 ELECTROLYTIC TREATMENT OF TIN PLATE FOR PREVENTING SULPHUR STAININ Arthur E. Stevenson, Oak Park, Ill., and Benton Hall Schaub, Long Beach, N. Y., assignors to Continental Can Company, Inc., New York, N. Y., a corporation of New York No Drawing. Application August 17, 1942 Serial No. 455,122

7 Claims.

This invention is concerned with the electrolytic treatment of tin plate for preventing staining by food products when packaged in containers having elements made of such tin plate.

When tin plate is employed for containers in packaging food-stuffs, such as vegetables, meat products and fish, decomposition of sulphur-containing proteins in the food products may occur, together with reaction with the tin during the sterilizing or processing of the products, resulting in discoloration or staining of the interior of the container. This discoloration is apt to be particularly troublesome in the head space if the products are slack-filled in the customary liquor or brine and subjected to prolonged processing. It is caused by the production of tin and iron sulphides, chiefly tin sulphide, which forms a film on the interior of the can.

It has been found that, by electrolytic treatment of tin plate as cathode in the presence of an alkaline bath containing chromate, a clear bright appearance can be procured and an immunity to sulphur-staining can be conferred thereon which permits non-staining employment of un-enameled 'tin plate in contact with foodstuffs which otherwise would undergo discoloration or staining. I

It has further been found that a more eflicient treatment can be quickly accomplished by producing a current reversal during the electrolysis, wherewith the tin plate is firstly made the anode during the treatment and then is made the cathode.

The electrolytic solution comprises a soluble chromate such as sodium or potassium chromate. It likewise contains an alkaline compound for assuring that the electrolyte shall remain alkaline throughout the treatment at a pH of substantially 8 to 12, usually around 11.3. The presently preferred alkaline compound is trisodium phosphate, and effective results have been obtained by employment of other alkalies such as sodium silicate, sodium hydroxide, sodium borate, sodium carbonate, and the corresponding soluble potassium salts, or in general soluble alkaline compounds which are capable of maintaining the electrolyte at a pH value in excess,

The concentration of chromate component and the concentration of alkaline component, likewise the relation of one to the other, may be varied over a, wide limit, but in general the concentration of alkaline component (on the anhydrous basis) should not be greater than that of the chromate (on the anhydrous basis) comiable results.

ponent, and it is preferable to keep the concentration of the former below that of the latter. For example, 2 to 25 grams per liter of anhydrous sodium chrornate, and 1 to 10 grams per liter of tri-sodium phosphate (dodecahydrate), may be used.

The time of treatment may be substantially 1 to 15 seconds, depending upon the current den-- sity, with preference for the lower times in commercial work, in order to provide a high throughput in employing the process. The surface effect is established very rapidly and an excess of time. i

protection against low-sulphur food-stuffs has been obtained in times as low as 1 second.

Although the solution temperature may be varied over a rather wide range with satisfactory results, it is advisable to keep it constant under any one set of operating conditions, particularly when can bodies are being treated. If this is not done, the heating of the solution, due to resistance to the passage of the current, causes a rapid rise in the amperage, thus producing var- For example, at a given concentration and a voltage of 50, and maintaining all conditions the same except temperature, the amperage varied from 22 at a temperature of 50 degrees F. to 47 at a temperature of 100 degrees F.

The current density necessary is directly proportional to the time of treatment. Where a high through-put is desired, it is advantageous to use a high current density. The amperage may vary from 10 to 100 per 100 square inches of tin plate surface depending upon the rate of production desired. The resistance effect within the electrolyte leads to a heating, and adequate precaution to maintain the temperature should be provided by way of using large bodies of electrolyte for immersion processes or by usingmeans for cooling the electrolyte during the treatment or while passing it from container to container in the flooding treatment.

The proportion of the time employed in anodic treatment, where the succession of anodic-cathodic steps is employed, should be less than that devoted to the final cathodic treatment. It has been found that when high current densities are used, from to of a. second of anodic treatment followed by 2% to 2% seconds of cathodic treatment with a total time of treatment of 3 seconds will give satisfactory results.

This electrolyte can be employed in a, vessel in which is immersed the tin plate or the container element of tin plate, or it can be employed for filling a container during the course of treatment.

The tin plate itself is connected for operation as an electrode in the electrolytic bath, and the other or counter-electrode can be provided by nickel, Monel metal, or other material which is not reactive with theelectrolyte to cause contamination thereof or to establish polarization or other effects requiring an excessive expendi-.

ture of current. It has been found that'the treatment can be accomplishedw'ith maintenance of a proper current density, by having the counterelectrode positioned at various distances from the tin plate. For example, in treating can bodies by filling the same with the electrolyte, the counter-electrode may be a centrally positioned nickel or Monei metal rod, or gauze of such materials, spaced up to 3 inches from the internal surface of the container, with employment of a 50 volt or lower potential and a current density of 10 to 100 amperes per 100 square inches of effective tin plate area, in a time of substantially 1 to 15 seconds, and at a temperature of 50 to 212 degrees F. In treating can ends, an adequate treatment can be efiected by using a stationary electrode of fiat sheet nickel which is positioned from ,4 to 11 inches from the surface of the tin plate, and in the same time.

When cans of various diameters are to be subjected to the treatment by flooding with the electrolyte and interposing an electrode, it has been found that cans having a diameter of around 3 to 3 inches can be adequately treated by a rolled nickel rod electrode having a. diameter of of an inch, and employing a change in voltage for different diameters of cans to assure similarity of treatment.

As specific examples of have been found eifective:

Example I The electrolytic solution was prepared with 10 grams of sodium chromate (anhydrous) and 2.5 grams of trisodium phosphate dodecahydrate (NaaP04.12H2O) per liter. Cans of sizes 303 x 406 (commercially known as No. 303 cans, having dimensions of 31% inches diameter and 4%; inches high, with the inside area of bottom and body about 45 square inches) and 307 x 409 (commercially known as No. 202 cans, and having dimensions of 31 inches diameter and 4 inches high, with the inside area of bottom and body about 50 square inches) were treated by employing a rolled nickel rod electrode having a diameter of inch and positioned the can, i. e. at a distance of 1% to 1 inches from the. internal surface thereof. The treatment consisted in making the can first the anode for V5 of a second and then the cathode for 2% seconds. The solution was maintained at '70 degrees F. at all times. The potential applied was about 45 volts for the 303 cans and about 50 volts for the 307 cans; being adjusted to obtain a current flow of around 60 amperes per 100 square inches of tin plate surface.

Can ends for these cans were treated by immersing them in an electrolytic solution of the same concentration and at the same temperature. No current reversal was effected in this instance and the ends were treated cathodically for 3 seconds with the use of a stationary electrode of flat sheet nickel positioned 5 inches from practice, the following concentrically in Example [I A solution containing sodium chromate equivalent to 7 grams of anhydrous sodium chromate per liter and trisodium phosphate equivalent to 5 grams of anhydrous trlsodium phosphate per liter was prepared. This was flooded into the can bodies, and electrolysis accomplished with a gauze counter-electrode having a diameter of 2 inches for 303 x 406 cans or 21% inches for 307 x 409 cans. The voltage employed was slightly less than 5 volts between the counte'relectrode and the can itself, giving current densities of approximately 40.3 and 39.7 amperes per square inches for the respective sizes of cans. The counter-electrode was operated only as anode, and was positioned close to the inner surface of the tin plate body. The time of treatment was 3 seconds.

Example III A solution containing 2 percent of sodium chromate (tetrahydrate) and 2 percent of sodium silicate (anhydrous metasilica) was employed with a tin plate as cathode with a temperature of around 70 degrees F., a current density of 30 amperes per square foot, and a time of treatment of 10 seconds. The tin plate was then resistant to sulphur staining.

Example IV do not have excessive sulphur staining effects,

in as short a time as 2 seconds. In these examples, the anode was a nickel wire mesh cylinder and the tin plate was in the form of can bodies positioned concentrically with the anode and spaced approximately $4; of an inch therefrom with the bottom end of the electrode open and positioned essentially the corresponding distance from the bottom of the can.

Example V The electrolytic solution used was prepared with 10 grams of sodium chromate (anhydrous) and 2.5 grams of tri-sodium phosphate decahydrate (Na3PO4.10H2O) per liter. Cans of size 307 x 409 were treated by employing 3, rolled nickel rod electrode having a diameter of inch and positioned concentrically in the can, i. e. at a distance of 1% to 1% inches from ti 1 internal surface thereof. The treatment consisted in making the can first the anode for of a second and then the cathode for 2% seconds. The solution was maintained at 212 degrees F. at all times. The potential applied was about 10 volts and the current flow was about'lO amperes per can, or 20 amperes per 100 square inches of tin plate surface.

The treatment gives the surface a clean appearance which is always at least as bright as,

but a condition has been established therein which prevents the tin plate from sulphur staining. The nature of the coating or effect of the treatment is not presently known, though there is indication that a chromium-containing film, possibly of molecular thickness, has been formed. The surface maintains its properties and is not aflected by exposure to air over a, long period of time. The treatment appears to have some effect in closing pores in the tin plate.

The presence of the alkali in the electrolyte appears to have an eilect differing from that of mere cleaning, by washing or immersion, as a pre-cleaning with an equivalent alkali solution does not produce the same effect.

The reversal of current which is employed as a phase of the treatment in Example I above has been found of great value. The preliminary anodic treatment is of shorter duration than the cathodic treatment, and can last from substantially A; to seconds under the concentrations and current densities stated, and with a, cathodic treatment time of about 2% seconds at a temperature of substantially '70 to 80 degrees F. and a pH of 11.3, with employment of a current density of 60 amperes per 100 square inches. The same applied voltages can be employed for both anodic and cathodic treatments, by use of a simple reversing switch. The product from such double treatment is superior to that which is obtained by employing chemical cleansing agents followed by the introduction of the tin plate to the cathodic treatment. The desired immunity is not attained even by lengthy treatment as an anode only.

The procedure is effective for treating tin plate generally, either in the form of sheets or webs, individual elements for providing bodies and ends of cans, or containers which are partially formed by joining the body seam or by body-seaming and providing one end.

It is obvious that the procedure can be practiced in many ways within the scope of the appended claims.

We claim:

to prevent sulphide discoloration when contacted with sulphur-containing products, which concludes with treating the same for substantially 1 to 15 seconds at a current density of substantially to 100 amperes per 100 square inches of effective tinplate area as cathode in an alkaline aqueous electrolyte containing soluble chromate in concentration of 2%; to 25 grams per liter computed as anhydrous sodium chromate, at a temperature of substantially 50 to 212 degrees F. and having a pH of substantially 8 to 12, said cathodictreatment producing upon the tinplate a surface of clear bright appearance resistant against staining by the sulphur-containing product.

2. The electrolytic method of treating a can body of tinplate to prevent sulphide discoloration when contacted with sulphur-containing products, which concludes with filling the body with an alkaline aqueous electrolyte containingsoiuble chromate in concentration of- 2 to 25 grams per liter computed as anhydrous sodium chromate, said electrolyte having a pH of substantially 8 to 12, and electrolyzing with the can body as cathode and a counter-electrode within the can body as anode at a current density of substantially 10 to 100 amperes per 100 square inches for a time of 1 to seconds at a temperature of substantially 50 to 212 degrees F.. said cathodic treatment producing upon the tinplate a surface of clear bright appearance resistant against staining by the sulphur-contain-' ing product.

3. The electrolytic method of treating tinplate to prevent sulphide discoloration when contacted with sulphur-containing products, which concludes with treating the same for substantially 3 seconds at a current density of substantially 40 amperes per square inches of eifective tinplate area, as cathode in an alkaline aqueous electrolyte at a temperature between substantially 50 and 212 degrees F. and containing sodium chromate in concentration of 7 grams per liter computed as anhydrous sodium chromate and5 grams of trisodium phosphate computed on the anhydrous basis, said cathodic treatment producing upon the tinplate a surface of clear bright appearance resistant against staining by the sulphur-containing product.

4. The electrolytic method of treating tinplate to prevent sulphide discoloration when contacted with sulphur-containing products, which concludes with treating the same for substantially 1 to 15 seconds at a current density of substantially 10 to 100 amperes per 100 square inches of efi'ective tinplate area, while maintaining a temperature between substantially 50 and 212 degrees F. in an alkaline electrolyte containing soluble chromate in concentration of 2 to 25 grams per liter computed as anhydrous sodium chromate and having a pH of substantially 8 to 12, a first part of the electrolytic treatment being effected with the tinplate as anode and the final part thereof being for a longer time and with the tinplate as cathode and therewith producing upon the tinplate a surface of clear bright appearance resistant against staining by the sulphur-containing product.

5. The electrolytic method of treating a can body of tinplate to prevent sulphide discoloration when contacted with sulphur-containing products, which concludes with filling the body with an alkaline aqueous electrolyte having solutes consisting essentially of sodium chromate in concentration of substantially 10 grams per liter computed as anhydrous sodium chromate and 2.5 grams of trisodium phosphate computed as dodecahydrate, electrolyzing with the can body as anode and a counter-electrode within the can body as cathode for substantially second, and

then reversing the current and electrolyzing with.

the can body as cathode and the counter-electrode as anode for a time of substantially 2% seconds, the two electrolytic treatments being effected at a current density of substantially 60 amperes per 100 square inches and with the electrolyte at a maintained temperature of substantially 70 degrees F., and therewith producing upon the tinplate a surface of clear bright appearance resistant against staining by the sulphur-containing product.

6. The electrolytic method of treating a can body of tinplate to prevent sulphide discoloration when contacted with sulphur-containing products, which concludes with filling the body with an alkaline aqueous electrolyte having solutes consisting essentially of sodium chromate in concentration of substantially 10 grams per liter computed as anhydrous sodiumchromate and 2.5 grams of trisodium phosphate computed as dodecahydrate, electrolyzing with th can body as anode and counterelectrode within the can body as cathode for substantially X; second, and then 1 reversing the current and electroiyzin with the can body as cathode and the counterelectrode as anode for a time of substantially 2% seconds, the two electrolytic treatments being eflected at a current density of substantially 20 amperes per 100 square inches with the electrolyte at a maintained temperature of substantially 212 degrees F.. and therewith producing upon the tinplate a. surface of clear bright appearance resistant against staining by the sulphur-containing product.

7. The electrolytic method of treating tinplate to prevent sulphide discoloration when contacted with sulphur-containing products, which concludes with treating the same for substantially Y5 second at a current density of substantially 60 amperes per 100 square inches of eflective tinplate area as anode in an alkaline electrolyte at a temperature of substantially '70 degrees F. and containing sodium chromate in concentration of substantially 10 grams per liter computed as anhydrous sodium chromate and having a pH of substantially 11.3, and thereafter electrolytically treating the same for a longer time as cathode at a temperature of 70 degrees F. in an alkaline electrolyte containing sodium chromate in concentration of substantially 10 grams 'per liter REFERENCES crrEn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,306,143 Stevenson Dec. 22, 1942 1,827,204 Mason Oct. 13, 1931 2,812,076 Cook et a1 Feb. 23, 1943 2,215,165 Sumner Sept. 17, 1940 2,314,818 Cook et al Mar. 23, 1943 1,734,706 Adler Nov. 5, 1939 OTHER REFERENCES Kerr, article in the Journal of the Soc. of Chemical Industry, Transactions and communi cations, Dec. 1940, macs 259-265. 

